In a DS-CDMA (Direct-Spread Code Division Multiple Access) transmission method, the frequency (spectral) efficiency is improved by synchronizing the receiving timings of reference paths among terminal stations at the base station thereby securing the orthogonality between the reference paths of the terminal stations that access to the base station simultaneously.
In the present document, “available (effective) path” means a path having a power level exceeding a predetermined threshold power level and “reference path” means a path having a highest power among the available paths or the first (coming) path, i.e., a path having the shortest arrival time.
As a method for maintaining the synchronization of the receiving timings of reference paths among the terminal stations at the base station, the adaptive transmission timing control method is known (Non-Patent Document 1).
A case where the adaptive transmission timing control method is applied in a conventional wireless communication system will be described below.
FIGS. 5(A) and (B) show delay profiles before and after the adaptive transmission timing control method is applied.
FIG. 5(A) shows the results when the delay profiles relating to two terminal stations 1 and 2 have been obtained at the base station.
The base station calculates an arrival time difference Tmax−T1, the difference between a maximum arrival time Tmax (a fixed value) assumed for an arrival path and an arrival time T1 of the reference path of the terminal station 1, and notifies the result to a terminal station 1 as a timing offset value.
Similarly, the base station calculates an arrival time difference Tmax−T2, the difference between the maximum arrival time Tmax and an arrival time T2 of a reference path of a terminal station 2, and notifies the result to the terminal station 2 as a timing offset value, followed by transmission.
The terminal stations 1 and 2 offset their transmission timings by adding these timing offset values to predetermined initial values, followed by transmission.
At this time, as shown in FIG. 5(B), the reference path timings of the terminal stations 1 and 2 are synchronized by the base station at the maximum arrival time Tmax.
When no available path having a power level exceeding the threshold value is detected at the base station, a method in which a path having a maximum power is defined as a reference path can be used as an adaptive transmission timing control method.
However, in the case where the path timing is not accurate, if one tries to move this path to the position of Tmax, the position of the right (true) path timing will greatly deviate from the searchable range and the path timing of the terminal station may not be found.
FIGS. 6(A) and (B) show delay profiles before and after the adaptive transmission timing control method is applied in a case where the paths are erroneously detected in the conventional communication system.
Here, we will assume that the transmission timing needs to be delayed so that the reference path arrives at the arrival time of Tmax shown in FIG. 6(A).
Since the arrival time Tmax is the maximum arrival time of signals assumed, arrival times later than this are not in the searchable range in designing of the wireless communication system.
FIG. 6(A) shows a case where a wrong path is selected as the reference path.
Referring to FIG. 6(B), if this erroneously selected path is moved to the position of Tmax, the right path will be outside of the searchable range and the base station will not be able to find the path of the terminal station.
[Non-Patent Document 1]
Kawamura et al., Technical Report of IEICE (Institute of Electronics, Information and Communication Engineers), Radio Communication System (RCS 2003-141), 2003, vol. 103, no. 363, pp. 13-18